This invention relates generally to gas turbine engines and, more particularly, to a thermally actuated control arrangement for maintaining minimum clearance between a rotor and surrounding shroud.
In an effort to maintain a high degree of efficiency, manufacturers of turbine engines have strived to maintain the closest possible clearance between the engine rotor and the surrounding stator structure, since any gas which may pass therebetween represents a loss of energy to the system. If a system were to operate only under steady-state conditions, it would be a simple matter to establish the desired close clearance relationship between the rotor and the stator to obtain the greatest possible efficiency without allowing frictional interference between the elements. However, in reality, all turbine engines must initially be brought from a standstill condition up to the steady-state speed, and then eventually decelerate to the standstill condition. This transitional operation is not compatible with the ideal low clearance condition just described since the variation in rotor speed also causes a variation in the size thereof because of mechanical expansion caused by centrifugal forces. The stationary stator, of course, does not grow mechanically and there is, therefore, relative mechanical growth between the two structures during periods of transitional operation. Further, as the turbine engine is brought up to speed from a standstill position, the temperature of the gas passing therethrough is increased proportionately, thereby exposing both the rotor and the stator to variable temperature conditions. These conditions cause thermal growth of the two structures, and if the two structures have different thermal coefficients of expansion, which is generally true, then there is also the occurrence of relative thermal expansion between the elements. Characteristically, a rotor is necessarily a large mass element which allows it to rotate at very high speeds, thereby inherently yielding a very slow thermal response (high thermal inertia). On the other hand, the stator is a stationary element and preferably has a high thermal response (low thermal inertia) to allow for thermal growth of the stator during periods of acceleration to accommodate the mechanical growth of the rotor during those periods.
In many turbine engine applications, there is a requirement to operate at variable steady-state speeds, and to transit between those speeds as desired in the regular course of operation. For example, in a jet engine of the type used to power aircraft, it is necessary that the operator be able to transit to any desired speed whenever he chooses. The resulting temperature and rotor speed changes therefore bring about attendant relative growth between the rotor and stator which must be accommodated for. As mentioned hereinbefore, a primary concern is to maintain the minimum clearance between the stator and rotor of the engine while preventing any frictional interference therebetween.
A typical cycle through which an aircraft jet engine operates begins with a "cold rotor burst" by which the engine transits from an idle operating condition to a maximum speed condition. The high thermal inertia rotor quickly grows by reason of its mechanical expansion, and slowly grows thereafter because of thermal expansion, until it reaches a steady-state diameter. The stator, on the other hand, grows quickly because of its relatively lower thermal inertia, to thereby provide room in which to allow the rotor to grow. Assuming that the jet engine reaches a steady-state maximum speed operating condition, the next speed transition may come about by a "throttle chop", by which the engine is again brought back to idle speed. When this occurs, the rotor immediately and quickly shrinks mechanically, and then continues to slowly shrink by reason of the change in temperature. The stator, on the other, hand experiences no mechanical shrinkage but begins to thermally shrink at a relatively fast speed. If the operation now calls for maximum throttle at the time the stator reaches its steady-state, reduced size, then the rotor will immediately mechanically expand to a larger size than when it experienced a "cold rotor burst." Since the stator has shrunk faster and farther than the rotor, it is during this period of operation that the clearance between the two elements is at a minimum and is therefore a critical criteria for the design of an aircraft jet engine. If the thermal response of the stator is reduced to cause a slower shrinkage thereof, and thereby accommodates a slower shrinkage of the rotor, then the required quicker expansion characteristics during periods of acceleration would be hampered. For example, if after the "throttle chop", the throttle is again brought up to maximum speed (hot rotor burst), then the stator must be capable of quickly expanding to accommodate the mechanical expansion of the rotor.
It is therefore an object of this invention to provide a gas turbine engine which is capable of transiting between various speeds while maintaining an allowable clearance between its rotor and stator.
Another object of this invention is to provide a high speed gas turbine engine with high efficiency characteristics during both steady-state and transitional operation.
Yet another object of this invention is to provide a gas turbine engine capable of operating over a variable speed schedule without attendant interference between the rotor and stator.
These objects and other features and advantages become more readily apparent upon reference to the following description when taken in conjunction with the appended drawings.